A protocol I like to use in my apps anywhere something like NSUserDefaults or KeyChain is normally used... allows for easy swapping of implementation. Also provided is StringDictionary() which can be used when testing

KeyValueStorable.swift

protocol KeyValueStorable {
    subscript (key: String) -> AnyObject? { get set }
    mutating func removeAll() -> Void
}

extension NSUserDefaults: KeyValueStorable {
    subscript (key: String) -> AnyObject? {
        get { return self.objectForKey(key) }
        set { self.setObject(newValue, forKey: key) }
    }
    func removeAll() {
        self.dictionaryRepresentation().keys.forEach { key in
            self.removeObjectForKey(key)
        }
    }
}
struct StringDictionary: KeyValueStorable {
    var dictionary = [String: AnyObject]()
    subscript (key: String) -> AnyObject? {
        get { return self.dictionary[key] }
        set { self.dictionary[key] = newValue }
    }
    mutating func removeAll() { dictionary.removeAll() }
}